As is known, as the technology evolves and devices for duplicating graphical elements which are printed or positioned on banknote paper become widespread, the need to have security elements which can be introduced at least partially in the banknote paper has been increasing and still is. One of the elements that is constantly being developed and is researched is the security thread that is generally inserted at least partially in banknote paper.
Over the years, the security thread has undergone a constant but significant evolution, so much that it is still one of the least counterfeited elements, since its counterfeiting is difficult even for expert counterfeiters.
Among security threads, it is possible to identify for example metallic threads with negative text, i.e., threads provided by a plastic substrate onto which at least one layer of metal is deposited, characters, lettering and the like being formed in such layer of metal by total removal of the metal at the regions of the characters and/or lettering.
EP 319 157 discloses the method outlined above.
A thread is also known for example in which, in order to increase the security characteristics, at least one full-surface layer is added which therefore covers both the regions without metal and the metallic regions with ink having fluorescent properties.
There are other types of threads in which discontinuities of the metal are inserted between blocks of letters so that conductivity can be detected in certain and known lengths.
However, the marketing of “transfer” metallized stripes has caused all these types of threads to undergo significant counterfeiting, which has led to the need to provide a partially demetallized thread, which thus leaves in the characters a small amount of metal, which can be detected by suitable laboratory equipment.
Therefore, in this last type of threads, the characters are demetallized only partially. Patent applications WO2004/014665 and WO2004/098900 relate to so-called partially demetallized threads of the type described above.
There are also types of threads which can be detected also by means of magnetic sensors. EP 516 790 discloses a thread in which detectable magnetic regions are inserted between the letters formed by performing total demetallization so that the thread can thus be detected both as conductive (due to the presence of metal deposited in continuous form) and by means of magnetic sensors (due to the magnetic elements arranged above or below the layer of metal but never inside the letters).
Therefore, EP 516 790 discloses a security device in which the characters or letters are totally demetallized.
Generally, coded magnetic threads are currently produced by depositing magnetic areas on a fully metallized polyester substrate; these areas are composed with a single type of magnetic ink and are separated by spaces in which regions without metallic material, meant for generating texts, are generated. It is evident that once the presence of magnetic areas has been discovered, their coercivity and residual magnetism can be identified easily and consequently so can the magnetic material to be used to create a counterfeit or forgery; a code thus provided generates the same signals, and therefore the same code, both when it is detected longitudinally (along the axis of the thread) and when it is detected transversely (reading at right angles to the thread).
Security threads with magnetic regions provided with a single type of magnetic ink also have undergone counterfeiting, again with the transfer technique. It is in fact possible to provide a ribbon or stripe which has negative characters by using metallic transfers onto which magnetic elements are transferred at a later time, between the blocks of letters, both in continuous form and in discontinuous form in order to create magnetic codes. To provide industrially a thread of the type cited above it is sufficient to have a rotary press with a plurality of printing sections as described above also in EP 516 790, printing onto a transparent material (generally polyester) markings with removable inks, performing full-surface vacuum metalization so as to cover the removable inks, removing the inks and consequently also the metal that covers them, thus leaving markings which are identical to the ones printed with removable inks, and subsequently reprinting with magnetic inks continuous or discontinuous areas in the regions which have not been demetallized and are therefore adjacent to the demetallized regions.
In this manner, the presence of magnetic elements is not visible with normal viewing instruments (naked eye, optical magnifying devices, et cetera), since they are always covered by metal. The magnetic elements are visible only by means of suitable devices dedicated to the detection of magnetism, such as for example magnetic scanners or lenses with liquid magnetic ink inserted in vacuum.